The present invention relates to the accurate measurement of sheet tension, and more particularly to the measurement of the tension experienced by a moving sheet at relatively small, localized portions of the entire sheet which is held under tension. Even more particularly, the present invention relates to the use of localized sheet tension measurements in the nondestructive determination of sheet strength. The present invention also relates to the non-destructive determination of the extensional stiffness of the sheet.
It is desirable to measure the tension of a moving sheet in a number of industrial sheet manufacturing and processing situations. A number of sheet tension measuring devices are disclosed in the art. With certain of these devices, the sheet is forcibly deflected from a straight path of travel between various parts of the sheet processing equipment. The force required to deflect the sheet and the amount of deflection are measured. A geometrical calculation is then performed to compute sheet tension, based upon the known deflection and applied deflecting force.
Although useful, there is a need for improvement of the above-mentioned sheet tension measuring devices. For example, to the best of applicants' knowledge, none of these devices securely hold the moving sheet to the surface of the sheet processing equipment at the points where the sheet is bent during the deflection. Thus, under certain situations, the sheet may tend to fly around somewhat and lift off of the surface of the equipment. This is particularly true when the sheet is moving at high speed, as would occur, for example, in modern paper mills. Moreover, wrinkles in the sheet will also tend to raise the sheet off of the surface of the equipment at the bending points. Thus, the deflection geometry may not be accurately known, and error is therefore introduced into the tension calculation.
Copending, commonly assigned U.S. patent application Ser. Nos. 730,406, filed May 2, 1985, 056,332, filed May 26, 1987, 195,364, filed May 13, 1988, 220,415, filed July 18, 1988, and 146,930, filed Jan. 22, 1988, each of which is incorporated herein by reference, disclose various devices and methods for nondestructively measuring the strength of moving sheet materials, particularly paper. As used herein, the term "strength" means the strength of the sheet at failure, and includes reference to such sheet strength parameters as the tensile strength, which is conventionally measured at the point where the sheet tears and also includes the well known "Mullen" sheet strength value which has conventionally been determined by a destructive sheet bursting test.
The major factors which influence the strength of, for example, paper sheet, are the basis weight of the sheet (i.e., the weight of the sheet per unit surface area) and the sheet thickness. These factors also effect the extensional stiffness of the sheet (i.e., the resistance of the sheet to being stretched). A nondestructive sheet strength sensor is disclosed in the cross-referenced patent applications, and one embodiment of this strength sensor is illustrated in the cross-sectional view of FIG. 1. By sensing a physical characteristic of the sheet related to extensional stiffness, this strength sensor 108 can be used to determine sheet strength.
The sheet strength sensor 108 of FIG. 1 uses a sheet support ring 12 which is split into 4 segments
12A-12D, each occupying approximately 90.degree. of the ring circle. A top view of this segmented ring 12 is illustrated in FIG. 2. Each segment 12A, 12B, 12C and 12D is supported on a pair of leaf springs 14. The freely rotatable wheel 18 forcibly deflects the moving sheet 20 into the center of the ring 12. It is preferred that the periphery of the wheel 18 be spherically convex rather than cylindrical. Each load cell 16A-16D senses the downward force of the moving sheet 20 on the corresponding supported ring segment 12A-12D, respectively. Since the wheel 12 bends the sheet 20 into the center of the ring 12, the output signal from each of the load cells 16A-D is dependent, in part, on the bending resistance of the sheet 20. Moreover, the portion of the sheet 20 passing under the wheel 18 must travel a greater distance than the sheet 20 which travels in a straight line outside of the ring 12. Thus, the sheet 20 is stretched by the wheel 18 and the loads sensed by load cells 16A-D are, therefore, also affected by the extensional stiffness of the sheet. As previously mentioned, the extensional stiffness can be related to sheet strength.
The 4 ring segments are aligned so that 2 segments 12A and 12C, are disposed on opposite sides of the ring 12 on a line oriented in the direction of travel of the sheet 20, as shown in FIG. 2. The orientations in both directions along the length of this line are known as the "machine directions". These "machine direction" ring segments are sensitive to the machine direction characteristics of the sheet 20. The remaining two ring segments, 12B and 12D, are sensitive to the "cross-direction" characteristics of the sheet 20. The "cross-directions" are perpendicular to the machine directions in the plane of the sheet. A computer (not shown in FIGS. 1-2) accepts the output signals of each of the 4 load cells 16A-D coupled to the ring segments 12A-D and calculates sheet strength using these signals in accordance with certain empirical equations which have been developed and are discussed below.
The process for making paper involves laying a wet mass of wood pulp fibers onto a moving porous belt, drying the mass, and finally calendaring the resulting paper sheet to give it the desired surface finish and thickness. The sheet strength sensor 108 is most advantageously used to monitor the strength of paper sheet after the final calendaring step, and before the paper is rolled-up on the final reel at the end of the manufacturing process. Since the strength of the paper produced may vary across the width of the sheet 20 as well as along the length of the sheet 20, the strength sensor 108 is preferably mounted on a scanning system, whereby the strength sensor 108 is scanned back and forth across the width of the sheet 20 in the cross directions while the sheet 20 is being fed out of the calendar and onto the final reel. In this way, the variations in sheet strength along the cross direction may be determined, as well as the strength of the paper sheet at each section along its length in the machine direction.
FIG. 3 illustrates a scanning system 22, which, as noted above, is preferably located after the final calendar rolls and before the final reel. In this figure, the sheet of paper 20 can be seen passing through the scanning station 22 in the direction of arrow 44 between two transverse beams 24 and 26. Upper and lower gauge support members, 28 and 30, are mounted to the upper 24 and lower 26 beams, respectively. The paper 20 sheet is shown in FIG. 3 with a cut out area so that the relationship between the gauge support members, 28 and 30, can be seen. A motor (not shown) within the scanning system 22 is coupled to and drives the gauge support members, 28 and 30, back and forth, in the cross directions in a continuous scanning motion, keeping the gauge support members, 28 and 30, in vertical alignment at all times.
As previously mentioned, the force exerted on each of the segments of the sheet supporting ring, 12A-12D, is a function of the extensional stiffness of the sheet 20 and the bending resistance of the sheet 20. The tension applied to the sheet 20 by the sheet processing equipment also affects the force on the ring segments 12A-12B. The extensional stiffness and the bending resistance of the sheet 20 may be different in the machine directions as compared to the cross directions, so that the force applied to load cells 16A and 16C is not necessarily the same as the force applied to load cells 16B and 16D. The difference in tension between the machine directions and cross directions also results in a different force sensed by load cells 16A and 16C as compared to load cells 16B and 16D. In the usual situation, as illustrated in FIG. 3, the sheet processing equipment provides tension only along the machine direction, to thereby cause the sheet 20 to move through the sheet processing equipment.
By properly combining the outputs of load cells 16A-16D, it is possible to make a nondestructive measurement of sheet strength on-line which accurately correlates with a wide variety of standard destructive sheet strength laboratory tests. For example, on-line measurements can be made of paper sheet which accurately correlates with conventional destructive laboratory tensile strength tests. To make such on-line measurements, the outputs of load cells 16A and 16C are fed to a computer along with the output of a displacement sensor 32 (FIG. 1) and a sheet tension measuring device 34 (FIG. 4). The displacement sensor 32 may be any one of a variety of known sensors, such as an eddy current device, which uses magnetic fields to determine the position of the wheel and hence the amount of deflection, Z, of the sheet. The sheet deflection value, Z, is used in the sheet strength equations discussed below.
There are also ways known in the art for measuring the tension in the sheet 20. One such device for measuring overall sheet tension is shown in cross section in FIG. 4. As shown in this figure, the sheet 20 is threaded around 3 rollers 36, 38 and 40, each of which extends across the entire width of the sheet 20. The rollers 36 and 38 are fixed relative to the paper-making machine and roller 40 is restrained from moving in a vertical direction by force transducer 42. The output of this force transducer 42 is a function of the average sheet tension and can be used to provide the sheet tension quantity, T, in the following equations.
Utilizing the equation for machine direction tensile strength disclosed in U.S. patent application Ser. No. 220,415, for example, the computer determines machine direction tensile strength, as follows: ##EQU1## where S.sub.md is the machine direction tensile strength of the paper sheet 20;
A, B, C, D and E are constants; PA1 L.sub.a and L.sub.c are the values of the output signals from the machine direction load cells, 16A and 16C, respectively, indicative of the downward force sensed by these load cells; PA1 T is a value representative of the average tension across the entire width of the sheet 20, as determined by the sheet tension measuring device 34; and Z is the output of the displacement sensor 32 indicative of the distance the sheet 20 is deflected into the ring 12. PA1 F, G, H, I and J are constants; PA1 L.sub.b and L.sub.d are the values of the output signals from the cross direction load sensors, 16B and 16D, respectively, indicative of the downward force sensed by these load cells; and PA1 T and Z are the same as discussed above in connection with equation (1). The constants F-J may be similarly calculated through conventional curve fitting techniques.
The constants A-E may be determined using conventional curve fitting techniques by correlating the results of conventional laboratory machine direction tensile strength tests with the value of S.sub.md.
The cross-directional tensile strength may be similarly calculated utilizing the following equation: ##EQU2## where S.sub.cd is the cross-directional tensile strength of the paper sheet 20;
Although the sheet tension measurement provided by the device of FIG. 4 provides a useful correction to the sheet strength measurement in cases wherein the overall sheet tension varies because of changes in the machine direction tension applied to the sheet by the sheet processing equipment, the device nevertheless measures tension across the entire sheet, and therefore may not provide readings representative of variations in the sheet tension being experienced by the relatively small portion of the sheet deflected by the wheel 18 of the strength sensor 108 at any particular instant. For example, it has been found that machine directionally oriented sheet tension can vary from point to point along the cross direction due to sheet edge effects. The reason for this variation is not completely understood. However, in papermaking, this difference in tension is thought to occur during the process of drying the sheet 20. During drying, the sheet fibers tend to shrink. Toward the middle of the sheet 20, the shrinkage is restrained by the surrounding fibrous material. This induces a tension in the fibers. However, there is less restraint at the sheet edges since there are fewer surrounding fibers. Here, the shrinkage is relatively unrestrained and, therefore, sheet tension is relieved near the edges of the sheet. As a result, machine directionally oriented sheet tension is, at least in certain instances, variable across the width of the sheet (i.e., in the cross-direction). Therefore, to provide the most accurate strength measurements at different positions across the width of the sheet, the sheet tension should be measured at different locations across the width of the sheet 20, and tension measurements from these different locations used in the sheet strength equations.